Quasars acting as gravitational lenses

The phenomenon of gravitational lensing is well known in the observation of galaxies by astronomers: the light emitted by a galaxy is distorted by the gravity of another galaxy that from our position is in front of the first. Using the Hubble Space Telescope, a team of astronomers has found several examples of gravitational lensing in which a galaxy acting as a gravitational lens contains a quasar.

Quasars are among the brightest objects known, so as to transmit a lot more light than entire galaxies that host them. The first quasars were discovered in the late ’50s using radio telescopes. In fact, the name quasar means “QUASi-stellAR radio source”.

For years, scientists debated the nature of quasars until they came to the conclusion that they are galactic nuclei containing supermassive black holes. Their brightness is caused by huge gas and other materials that fall into these black holes forming an accretion disk. The amount of matter falling towards the black hole can be enormous and the gravitational energy they emit causes their huge brightness.

The quasar-galaxy combinations searched by astronomers are rare so the team that’s been conducting this research selected 23,000 quasars spectra from the Sloan Digital Sky Survey (SDSS). This is a mapping project born in the late nineties, a collaboration between Americans and Japanese to study a part of the sky up to a distance of one and a half billion light years from Earth.

To find the right candidates, astronomers searched for alignments between galaxies and the one identified were observed with the Hubble Space Telescope. In particular, in the images captured using Hubble’s Wide Field Camera 3 they searched for gravitational arcs and rings such as those indicated by the arrows in the image which indicated the existence of a gravitational lens.

Generally it’s difficult or even impossible to see the galaxies in which there’s a quasar because their light is too weak compared to the quasar. The consequence is that it’s difficult to estimate the mass of the galaxy based on the overall brightness of its stars. Instead, using gravitational lenses they can estimate the mass of a galaxy that hosts a quasar based on its gravitational effects.

Now, astronomers want to create a catalog of gravitational lenses created by quasars. These data will be used to improve our understanding of the relationships between the way supermassive black holes absorb matter and star formation in the evolution of galaxies.